Extruded tubes and cable sheaths



April 16, 1963 P. BILLEN ET AL EXTRUDED TUBES AND CABLE SHEATHS Filed Oct. 24, 1957 IN l/EN TO ITS LZ 817/ flHaezbd elem Schloemann Aktiengesellschaft, Dusseldorf, Germany Filed Oct. 24, 1957, Ser. No. 692,230 2 Claims. (Cl. 207-) The invention relates to a method, by the relative displacement of mandrel and die, of preventing the development of the annular markings known as bamboo rings on extruded tubes and cable sheaths which are cooled immediately they leave the die. According to an earlier proposal (German patent application Sch 14,000 Ila/7b, of November 1953, which issued as Patent No. 1,012,892; U.S. application Ser. No. 460,961, which issued as Patent No. 2,828,859 on April 1, 1958, of October 7, 1954; British Patent No. 765,367, published Jan. 9, 1957) the normal velocity of extrusion is gradually reduced to nil in such a way that during the standstill period the cooling cable sheath and the tube-forming tools will at no time assume temperatures likely to injure the insulation of the cable. As the temperature and the speed of extrusion go down, the die will necessarily contract and reduce the size of the annular orifice through which the sheath is extruded, so that the thickness of the wall of the tube will slowly diminish. Moreover, the gradual reduction of the normal velocity of extrusion involves a concomitant reduction in pressure. The drop in pressure affects the pressure inside the extrusion chamber behind the orifice of the die. Consequently there is some recovery from the longitudinal elastic strain imposed by the normal pressure of extrusion, and this recovery likewise affects the extrusion chamber itself. Hence, the thermal variation of the size of the annular orifice due to the change in the temperature will be modified by an elastic variation due to the changing extrusion pressure and the consequent fluctuation of the load on the tools. The magnitude of this elastic variation depends upon the amplitude of the pressure fluctuation that occurs in the space con taining the tools. If the pressure of extrusion is abruptly relaxed, then a pronounced annular depression will form on the extruded tube, known as a bamboo ring. If the drop in pressure is gradual, then the resultant marking will not be pronounced. Whereas rapid cooling causes the diameter of the die to become smaller by radial contraction and hence entails a reduction in the wall thickness of the cable sheath, the withdrawal of heat from the tool head by the cooling of the tube when the standstill periods are long causes a shrinkage of mandrel and die also in the axial direction. This axial shrinkage involves an enlargement of the annular extrusion orifice and hence tends to thicken the wall section of the extended tube.

It has already been disclosed that the depression which forms part of the bamboo ring and which is due to elastic changes in the size of the extrusion orifice at the end of a pressure period can be prevented from forming by a displacement of the mandrel, i.e. by an adjustment of the size of the orifice, i.e. a variation of the distance between mandrel and die. The object taken in view by the present invention is to eliminate the eifect of changes in the size of the extrusion orifice due to causes that had not heretofore been considered, namely those due to changes in temperature. When extruding lead sheaths these temperature variations do not produce considerable and particularly troublesome changes. However, when extruding aluminium, the temperature fluctuation that must be controlled covers a range of several hundred degrees C. and this is rapidly traversed owing to the high rate of cooling the process requires, which therefore produces very sensible thermal shrinkage effects. Hence the Patented Apr. 16, 1963 present invention is more especially concerned with the extrusion of aluminium and aluminium alloys. The method proposed by the present invention is based on the earlier method of preventing, by means of the relative displacement of mandrel and die, the development of the annular markings known as bamboo rings on extruded tubes and cable sheaths of aluminium or other metals which are subjected to cooling immediately they leave the die, the said earlier method consisting in displacing the tools to increase the size of the annular extrusion orifice when the pressure drops and to reduce the size of the orifice when pressure is re-applied. The method now proposed by the present invention, in which the adjustment of the tools to increase the size of the annular orifice is effected before the standstill period begins, is primarily distinguished from the aforementioned earlier proposal by the fact that it is performed in a gradual manner in proportion with the gradual contraction of the annular orifice due to the cooling of the die. At the same time the simultaneous contraction of the annular orifice due to the gradual drop in the pressure can likewise be taken into consideration (as proposed in German patent application Sch 14,000 lb/ 7b, U.S. patent application Ser. No. 460,961, and

British Patent No. 765,367). Another feature of the invention is that the tools are displaced in the sense of reducing the size of the orifice before the press is restarted at the end of a standstill period. The amount of displacement is preferably calculated to allow for the degree of thermal contraction of the extruded material which is within the region of the die during the standstill period of the press. Allowance may also be made for contraction due to the pressure drop that precedes the standstill period of the press.

Yet another feature of the invention is that the further adjustment of the tools in the sense of reducing the size of the orifice until they have resumed the position which corresponds with normal extrusion velocity after the press is re-started is gradually performed approximately in proportion with the increase in the size of the extrusion orifice due to the die again heating up.

The accompanying drawings are axial sections of parts of three extruded aluminium cable sheaths showing the inequalities (bamboo rings) in the conformation of the sheaths at the points which correspond with the standstill periods of the press. The sheath shown in FIGURE 1 is produced without any displacement of the extrusion tools and therefore exhibits the development of a pronounced bamboo ring. In the production of the sheath shown in FIGURE 2 the size of the annular extrusion orifice was increased as the pressure fell at the end of the extrusion stage before the standstill period began and it was reduced again when pressure was re-applied. FIG- URE 3 illustrates the method of controlling the extrusion orifice as proposed by the present invention.

In the method illustrated in FIGURE 1 the pressure of extrusion is gradually reduced before the point 1, which corresponds with the standstill period, is reached,

so that the wall thickness d of the extruded sheath will likewise diminish from the left to the right towards point '1. During the standstill period the die cools and 'therefore radially contracts, thus pressing a bamboo ring, in the form of a notch-like annular groove, into the wall of the sheath. For re-starting the press a high pressure is then applied, which causes the size of the orifice to increase and the tube wall to thicken abruptly at the point 2. During the ensuing pressure stage the temperature of the die gradually rises and the thickness of the tube wall slowly increases to assume its original value d at about the point 3. The resultant confonnation of the wall, as shown in FIGURE 1, is therefore non-uniform and unsatisfactory.

In the method illustrated in FIGURE 2, which is an improvement on that of FIGURE 1, the wall thickness likewise gradually diminishes from its normal value d on the left to a smaller value ti at the standstill point 4- owing to the gradual pressure drop during this period. However, when the press stops, at the point 4, the tools are adjusted in the sense of increasing the width of the orifice. This adjustment therefore compensates the impending reduction in wall thickness due to the contraction of the die. When the press is re-started, at the point 5, the pressure of the extruded material rises again and the thickness of the wall tends to increase, but this is now compensated by a readjustment of the tools, in the sense of decreasing the orifice, at point 6. In other words, the tools are first displaced, at the point 4, to increase the orifice, and they are then readjusted, at the point 6, to reduce the orifice. This method is better than the method illustrated in FIGURE 1, because there is now no abrupt reduction in wall thickness as at 1 in FIGURE 1, but rather a short increase in thickness. The further course of the extrusion process from point 6 corresponds with that shown in FIGURE 1, and the original wall thickness d will therefore be re-established.

In the method now proposed by the present invention, which is illustrated in FIGURE 3, the pressure again gradually diminishes from the left to the standstill point with a concomitant reduction in the velocity of extrusion. The efiect of cooling would therefore be to diminish the wall thickness accordingly. However, according to the invention it is proposed to counteract this gradual decrease from the very beginning by displacing the tools at the point 7, where the reduction of pressure begins in the sense of progressively increasing the orifice so that the wall thickness will not actually diminish at all but will maintain its original value d until the standstill point is reached, or may even be slightly increased. During the standstill period of the press, which begins at point 8 the tools are re-adjusted at the point 9 for the further enlargement of the extrusion orifice to compensate for the thermal contraction of the die. During the possibly prolonged standstill period the extrusion material in the vicinity of the orifice will cool so that, roughly expressed, it will lie between the tools with play, thus tending to give rise to a sudden considerable increase in wall thickness when pressure is re-applied. T o prevent this from happening it is proposed by the invention to re-adjust the tools at the point 10 during the standstill period in the sense of reducing the orifice.

When pressure is resumed at the end of a prolonged period of standstill the increase in wall thickness, in-

dicated at points 9 and 10, will therefore be only slight (wall thickness greater than or equal to d) and this is then compensated up to point 11 by the further adjustment of the tools to constrict the orifice, thus at the same time continuously compensating the increase in wall thickness that will be caused by the rise in the temperature of the die, and therefore operating to maintain the wall thickness d. Hence, besides preventing the development of depressions, the present invention will also reduce any undesirable thickening of the wall. If the adjustment of the tools is skillfully controlled and performed, any thickening of the wall may even be entirely avoided. However, whenever such thickening does occur, the transitions will not be abrupt, a factor which will tend to impart to the tube or sheath a much greater flexural strength on the bending machine than when changes in wall thickness are well defined and abrupt.

Although the variations in the thickness of the walls may not always be as pronounced as shown in the drawings, the individual sections are nevertheless easily detectable when the cable sheath has been completed.

Since in actual practice it is very rarely possible by adjusting the tool to compensate with entire precision shrinkage due to changes in temperature as well as elastic changes due to fluctuations in pressure, it is preferred in making the adjustments to be somewhat generous in the positive direction i.e. in the sense of an increase in the thickness of the sheath wall beyond its thickness in other regions, and at points 7 and 11 not to effect the adjustment abruptly but gradually whilst the sheath still slowly moves through the die. A small and uniform temporary increase in the wall thickness throughout the controlled length of the cable is an advantage when, as is frequently necessary, the standstill periods are long, because the increase will then offset the effect of metallurgical changes due to cooling, which would otherwise adversely affect the flexural properties of the cable.

What is claimed is:

1. A method of controlling the thickness of a sheath or tube of aluminium or other metal produced in an extrusion press in which the thickness is determined by the distance between one end of the mandrel and the end face of the die aperture and in which the extruded product is cooled immediately after leaving the die, comprising the steps of reducing the extrusion pressure gradually when a stoppage is to be effected, progressively adjusting the relative axial spacing of the mandrel and die from the beginning of the reduction in pressure and during its continuance to the extent necessary to compensate for the reduction in the annular gap that would otherwise result from the variation of the load on the mandrel and die caused by the changing extrusion pressure, continuing the adjusting of the mandrel and die to compensate for the reduction in the annular gap that would otherwise be caused by the cooling and consequent contraction of the die, and progressively re-adjusting the mandrel and die when extrusion is resumed, to the extent necessary to compensate for the thermal and elastic expansion of the die resulting from the increase in temperature and pressure.

2. A method of controlling the thickness of a sheath or tube of aluminium or other metal produced in an extrusion press as claimed in claim 1, further comprising the step of reducing the size of the extrusion gap before the press is re-started after a standstill period to the extent necessary to compensate for the thermal contraction of the cooling extrusion material in the vicinity of the die during the stand-still period.

References Cited in the file of this patent UNITED STATES PATENTS 2,131,173 Greenall Sept. 27, 1938 2,177,681 Anderson Oct. 31, 1939 2,782,920 Norman Feb. 26, 1957 2,828,859 Emmerich Apr. 1, 1958 FOREIGN PATENTS 380,037 Great Britain Sept. 8, 1932 876,573 France Oct. 10, 1942 909,332 Germany Apr. 15, 1954 OTHER REFERENCES Advanced Fluid Mechanics, vol. 1, by R. C. Binder, Prentice-Hall, Inc., Englewood Cliffs, N.J., 1958, pp. 263-271. 

1. A METHOD OF CONTROLLING THE THICKNESS OF A SHEATH OR TUBE OF ALUMINUM OR OTHER METAL PRODUCED IN AN EXTRUSION PRESS IN WHICH THE THICKNESS IS DETERMINED BY THE DISTANCE BETWEEN ONE END OF THE MANDREL AND THE END FACE OF THE DIE APERTURE AND IN WHICH THE EXTRUDED PRODUCT IS COOLED IMMEDIATELY AFTER LEAVING THE DIE, COMPRISING THE STEPS OF REDUCING THE EXTRUSION PRESSURE GRADUALLY WHEN A STOOPAGE IS TO BE EFFECTED, PROGRESSIVELY ADJUSTING THE RELATIVE AXIAL SPACING OF THE MANDREL AND DIE FROM THE BEGINNING OF THE REDUCTION IN PRESSURE AND DURING ITS CONTINUANCE TO THE EXTENT NECESSARY TO COMPENSATE FOR THE REDUCTION IN THE ANNULAR GAP THAT WOULD OTHERWISE RESULT FROM THE VARIATION OF THE LOAD ON THE MANDREL AND DIE CAUSED BY THE CHANGING EXTRUSION PRESSURE, CONTINUING THE ADJUSTING OF THE MANDREL AND DIE TO COMPENSATE FOR THE REDUCTION IN THE ANNULAR GAP THAT WOULD OTHERWISE BE CAUSED BY THE COOLING AND CONSEQUENT CONTRACTION OF THE DIE, AND PROGRESSIVELY RE-ADJUSTING THE MANDREL AND DIE WHEN EXTRUSION IS RESUMED, TO THE EXTENT NECESSARY TO COMPENSATE FOR THE THERMAL AND ELASTIC EXPANSION OF THE DIE RESULTING FROM THE INCREASE IN TEMPERATURE AND PRESSURE. 